Device for carrying out a plasma-assisted process

ABSTRACT

A device for carrying out a plasma enhanced process includes, within a vacuum chamber, at least one magnetron electrode ( 32 ) constituting an unbalanced magnetron having a flat magnetron face ( 20 ) with peripheral and central magnetic poles of opposite polarities connected to a source ( 34 ) of alternating voltage. The device further includes a device for positioning a substrate ( 25 ), the substrate having a surface to be treated facing the magnetron face ( 20 ), and a gas supply device for supplying a process gas or process gas mixture to the space between the magnetron face ( 20 ) and the treated surface. The distance between the magnetron face ( 20 ) and the treated surface is adapted to the magnetic field created by the magnetron electrode ( 32 ) such that there is a visible plasma band running between darker tunnels formed by magnetic field lines extending between peripheral and central magnetic poles of the magnetron face ( 20 ) and the treated surface, the plasma band having a minimum width but having homogeneous brightness towards the treated surface.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a device for carrying out a plasma enhancedprocess, in particular a plasma enhanced chemical vapour deposition. Thedevice serves e.g. for coating one side of a web of film or sheetmaterial, in particular for coating a web of polymer film with siliconoxide in order to improve its barrier properties.

2. Description of Related Art

It is known e.g. from the publication EP-299754 (BOC) to deposit a thinfilm of silicon oxide on a substrate in a plasma enhanced chemicalvapour deposition process. According to this publication a vacuumchamber is provided in which the electrically isolated substrate ispositioned to face a magnetron being powered with an alternatingvoltage. A stream of a process gas mixture consisting of anorganosilicon compound, oxygen and an inert gas (e.g. argon or helium)is flown into the space between the face of the magnetron and thesubstrate and the plasma derived from the process gas mixture ismaintained at a pressure of e.g. 6 Pa.

The magnetron described in the publication EP-0299754 is a flatmagnetron of a balanced or unbalanced type. The degree of unbalance of aflat magnetron depends on the strength ratio of the magnetic pole orpoles positioned on either side of the track running around themagnetron face, i.e. on the ratio between the number of magnetic fieldlines extending from north to south poles across the track to the numberof field lines not doing so. The unbalanced magnetron is known to notfully confine electrons and ions of the plasma such that there is alimited amount of electron and ion bombardment of the substrate which issaid to improve the deposition quality. The magnetron face of theunbalanced magnetron according to EP-0299754 comprises peripheral northpoles and a central core of soft iron, resulting in an only smallportion of the field lines extending from the north poles to the core(highly unbalanced magnetron).

Publication EP-0605534 (BOC) describes a similar plasma enhancedchemical vapour deposition process wherein the substrate is a web. Theweb is carried by a rotating drum which constitutes the poweredelectrode and is negatively biased. Facing the web carried by the drumthere is an electrically grounded and possibly cooled shield the backside of which is faced by at least one pair of opposing magnetic poles,preferably a series of alternating magnetic poles. Aided by the magneticfield the plasma is confined between the drum and the shield. Theadvantage of the arrangement is seen in the decoupling of the electricand the magnetic field, which is said to lead to an extension of theplasma throughout the plasma volume (between drum and shield). Thedistance between drum and shield is described as having to be within therange of 1 to 30 cm.

SUMMARY OF THE INVENTION

The object of the invention is to improve the above named devicesapplicable for plasma enhanced processes, in particular for plasmaenhanced chemical vapour deposition but also for e.g. plasma etching orplasma processes for changing the wetability or adhesion characteristicsof a surface and belonging to the type being based on a magnetron. Theimprovement is to regard in particular process efficiency. For adeposition process deposition rate and deposition quality are to beimproved.

The device according to the invention comprises a vacuum chamber andwithin the chamber at least one magnetron electrode comprising anunbalanced magnetic pole arrangement and means for positioning asubstrate with a surface to be treated facing the magnetron electrodeand serving as counter electrode. Either electrode is powered with analternating voltage. Advantageously the magnetron electrode is poweredand the substrate or the positioning means carrying the substrate iselectrically grounded, electrically floating or negatively biased. Thedistance between the magnetron face and the surface of the substrate tobe treated is adapted to the characteristics of the magnetic fieldcreated by the permanent magnet poles of the magnetron face, beingdefined mainly by the magnetic strength of the poles, by the degree ofunbalance of the pole arrangement and by the width of the track betweenthe magnetic poles.

Experiments with a plasma enhanced chemical vapour deposition process inan otherwise unchanged system (same magnetic field and same electricfield) show that the deposition rate is dependent on the distancebetween the magnetron face and the substrate in such a way that anoptimum regarding deposition rate and deposition quality is found whenthe surface to be coated is positioned just outside the tunnels formedby the magnetic field lines extending across the track on the magnetronface, i.e. when the distance between magnetron face and surface to becoated is only slightly larger than the extension (height) of thetunnels from the magnetron face. Preferably the distance between thesurface to be coated and the magnetron face is by 2 to 20% larger thanthe tunnel height. Within this range, the surface to be coated ispositioned in an area where the electron density is higher than in thetunnels but where a considerable portion of the magnetic field lines arestill shaped by the tunnels, i.e. have a component parallel to themagnetron face.

Visual observation of a plasma maintained between an unbalancedmagnetron with a flat rectangular face (observation parallel to thelonger side of the magnetron face) and a surface to be treated andarranged substantially parallel to the magnetron face shows the namedtunnels as quite clearly distinguishable darker areas within the plasma,which outside of the tunnels appears brighter. The above discussedsetting of the distance between the magnetron face and the substrate tobe dependent on the characteristics of the magnetic field can easily bebased on such observation. The surface to be coated is positioned beyondthe tunnels such that there is a bright plasma band extending betweenthe tunnels and the surface to be treated, the band having a minimumwidth but having towards the surface to be coated a homogeneousbrightness, i.e. the same brightness at positions above tunnels as at aposition above the gap between tunnels.

BRIEF DESCRIPTION OF THE DRAWINGS

The principle of the device according to the invention as well as apreferred embodiment thereof are described in further detail inconnection with the following figures, wherein:

FIG. 1 shows the principle of the device according to the invention;

FIGS. 2 and 3 show an exemplified magnetron electrode applicable in thedevice according to the invention (FIG. 2: top view of the magnetronface; FIG. 3: section perpendicular to the magnetron face, section lineIII-III in FIG. 2);

FIG. 4 shows a preferred embodiment of the device according to theinvention, serving for coating a web of flexible material, e.g. forcoating a polymer film material with silicon oxide for improving itsbarrier properties.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows the magnetic field of a flat unbalanced magnetron in asection perpendicular to the magnetron face. Three permanent magnets 1are arranged alternately, their poles on one side being connected by apiece 2 of a magnetizeable material, e.g. soft iron. The poles oppositethe connecting piece 2 form the flat magnetron face (plane A) whichcomprises e.g. a central north pole and peripheral south poles, thenorth pole e.g. having the same strength as each one of the south poles.There are further provided means (not shown) for establishing analternating electric field with electric field lines extendingsubstantially perpendicular to the magnetron face, e.g. an electrodepiece extending over the magnetron face and being powered by analternating voltage (see FIG. 3).

The pole arrangement of the face of an unbalanced magnetron creates afirst portion of magnetic field lines 10 extending from north pole toeither one of the south poles such forming the tunnel 11 on the base ofwhich electrons and ions are confined and another portion of field lines10′ originating elsewhere and ending in the south poles. The extensionof the tunnels 11 (plane B) above the magnetron face is dependent on themagnetic strength of the poles, on the distance between the poles (widthof track) and on the ratio of the strengths of central and peripheralpoles (degree of unbalance of the magnetron).

According to the invention, the substrate surface to be treated (planeC) is positioned such that it is definitely outside of the tunnels 11but as near as possible to the magnetron face A. The distance betweenplanes A and C is preferably at least 2% larger than the distancebetween planes A and B, even more preferably between 2 and 20% largerthan the distance between planes A and B.

FIGS. 2 and 3 show an exemplified embodiment of a magnetron electrodeapplicable for the inventive device. The magnetron electrode has a flat,rectangular face and is again of the unbalanced type. FIG. 2 shows theface of the magnetron, FIG. 3 shows a section perpendicular to themagnetron face (section line III-III of FIG. 2) and a substratepositioned facing the magnetron electrode e.g. for being coated.

The magnetron electrode comprises alternately arranged permanent magnets1. A peripheral arrangement of north poles and a central line of southpoles (or five bar shaped permanent magnets with opposite poles onopposite longitudinal sides) constitute the magnetron face 20 which iscovered by a powered electrode piece 21 being made of a nonmagnetizeable material, e.g. of aluminium, stainless steel or copper andbeing connected to a source 22 of a high frequency alternating voltage.The same material is preferably used for filling the gaps 23 between thepermanent magnets 1. The magnetic poles facing away from the magnetronface are connected by a connecting piece 2 of a mangetizeable materiale.g. of soft iron. A peripheral wall 24 surrounding the permanentmagnets is preferably made of a magnetizeable material also.

The substrate to be treated is e.g. a web 25 being supported by agrounded support 26, along which the web is moved continuously (arrow28). The support 26 may also be electrically floating or negativelybiased. The plasma is confined between the magnetron face and thesubstrate, optimum deposition being achieved with a distance A-Cfulfilling the above described conditions. The process gas mixture isflown through the plasma space, e.g. in the manner as illustrated byarrows 27.

FIG. 4 shows an exemplified embodiment of the inventive device, whichembodiment serves for coating or otherwise treating a web of a flexiblesubstrate material. The substrate 25 is transported by a rotating drum30 constituting the support 26 for the substrate 25. The web is unwoundfrom a first supply roll 31 and wound onto a second supply roll 31.Around part of the circumference of the drum 30 and at the abovedescribed distance from it, a plurality of magnetron electrodes 32 asillustrated by FIGS. 2 and 3 is arranged. The magnetron faces facetowards the drum 30 and their length is arranged parallel to the drumaxis. Gas supply lines 33 (e.g. tubes comprising a line of gas supplyapertures) for supplying the process gas mixture are arranged extendingparallel to the drum axis between the magnetron electrodes 32. Thearrangement of drum 30, supply rolls 31, magnetron electrodes 32 and gassupply lines 33 is arranged in a not shown vacuum chamber equipped withmeans for removing gas from the chamber and keeping the inside of thechamber at a constant reduced pressure. Each magnetron electrode 32 isadvantageously electrically powered by its own power supply 34. Theprocess gas flows mainly from the supply lines 33 towards the drum facesfrom where it is evacuated.

Experiments show that using an arrangement with a plurality ofindividually powered magnetron electrodes 32 not only makes operationmore reliable (operation with one defect magnetron electrode can becarried on with a correspondingly reduced web speed) but also improvesefficiency. While an arrangement as shown in FIG. 4 produces a visiblyhomogeneous plasma along the web surface to be coated and highoperational stability, a similar arrangement with a singly poweredmagnetron-like arrangement as e.g. shown in the publication EP-0605534spanning a similar sector of the drum circumference shows a gradient inplasma intensity along the space between drum and shield with anincreasing intensity from web entry to web exit and with a very highplasma intensity in the area of the web exit, constituting a source ofinstability.

EXAMPLE 1

A device according to FIG. 4 with four magnetron electrodes according toFIGS. 2 and 3, each magnetron face being 600 mm long and 150 mm wide andeach magnetron face comprising a central permanent magnet of a magneticinduction of ca. 100 Gauss (10⁻² Tesla) and peripheral permanent magnetsof ca. 200 Gauss being arranged around the central magnet with adistance between the poles of ca. 50 mm is used for coating a polymerfilm with silicon oxide using a plasma derived from a process gasmixture comprising an organosilicon compound and oxygen. The magnetronfaces are positioned at a distance from the drum circumferential surfaceof ca. 60 mm (visibly such that there is a narrow bright band of plasmarunning beyond the tunnels along the substrate showing towards thesubstrate a regular intensity not being dependent on the tunnelpositions). The magnetrons are powered with a total of 14 kW per m² ofmagnetron face at a frequency of 40 kHz. The deposition rate suchachieved is ca. 3 nm per second with a high barrier quality and highreliability. Alterations of the distance between magnetron faces anddrum circumferential surface in either direction result in a relevantreduction of the deposition rate.

Using a similar set up and similar operation parameters except for thepermanent magnets having a four times higher magnetic induction resultsin a deposition rate maximum at a distance between magnetron faces anddrum circumferential surface which is lager than 60 mm, preferablybetween 80 and 100 mm and in a deposition rate clearly above thedeposition rate achieved with the weaker magnets as described above.

1. A device for carrying out a plasma enhanced process, the devicecomprising within a vacuum chamber a magnetron electrode (32), apositioning means and a gas supply means, the magnetron electrodecomprising a flat magnetron face (20) with peripheral and centralmagnetic poles of opposite polarities and further comprising means forproducing a high frequency alternating electric field, the positioningmeans being equipped for positioning a substrate (25) with a surface tobe treated facing the magnetron face (20) and the gas supply means beingequipped for supplying a process gas or process gas mixture to the spacebetween the magnetron face (20) and the surface to be treated, whereinthe magnetron electrode (32) is of the unbalanced type and that adistance between the magnetron face (20) and the positioning means isadapted to the magnetic field created by the magnetron electrode (32)such that there is a visible plasma band running between darker tunnels(11) formed by magnetic field lines (10) extending between peripheraland central magnetic poles of the magnetron face (20) and the surface tobe treated, the plasma band having a minimum width but having towardsthe surface to be treated a homogeneous brightness.
 2. The deviceaccording to claim 1, wherein a distance (A-C) between the surface to betreated and the magnetron face (20) is at least 2% larger than a visibleheight (A-B) of the tunnels (11).
 3. The device according to claim 1,wherein a distance (A-C) between the surface to be treated and themagnetron face (20) is at most 20% larger than a visible height (A-B) ofthe tunnels (11).
 4. The device according to claim 1, wherein a magneticstrength of the central magnetic pole of the magnetron face (20) isabout half of a magnetic strength of the peripheral pole.
 5. The deviceaccording to claim 1, wherein the magnetron electrode (32) comprises anelectrode element (21) being connected to a source of an alternatingvoltage (34).
 6. The device according to claim 5, wherein thepositioning means and/or the substrate (25) are arranged to beelectrically grounded, electrically floating or negatively biased. 7.The device according to claim 1, wherein the positioning means is arotating drum (30) and wherein a plurality of magnetron electrodes (32)having rectangular faces arranged with their length parallel to therotation axis of the drum (30) are arranged around part of acircumference of the drum (30).
 8. The device according to claim 7,wherein the gas supply means comprises gas supply lines (33) extendingparallel to the drum axis between the magnetron faces (20).
 9. Thedevice according to claim 7, wherein each of the plurality of magnetrons(32) is connected to a separate power supply.
 10. Use of the deviceaccording to claim 1 for carrying out a plasma enhanced chemical vapourdeposition process.
 11. Use of the device according to claim 1 fordepositing silicon oxide using a process gas comprising an organosiliconcompound and oxygen.
 12. Use according to claim 11, wherein thesubstrate is a web of polymer film material being coated so as toimprove barrier properties of said web of polymer film material.